Piston rings is one of the key components of the internal combustion engine, thus its wear and tear is a very significant for the overall fuel consumption of the engine. At present, vehicle engines (particularly diesel engines) is developing in the direction of high mechanical loads, high output power (high heat load), low fuel consumption and low exhaust emissions. Therefore the piston ring become thinner in thickness and lighter in weight, which requires high strength (thermal fatigue strength), abrasion resistance, pull-cylinder performance for the piston ring, as well as the sliding characteristics of no much wear of the cylinder sleeve. Advanced surface treatment technology has been vigorously promoted the popularized in the field of the piston rings. In recent years, the steel piston rings previously often used in gasoline are gradually introduced to the field of diesel engines and the steel ring began to be used in some diesel engines. At present, more advanced nitriding, spraying molybdenum and PVD technologies have been widely adopted for the steel ring.
During the engine running, the piston ring of the engine is subjected to not only the impact of thermal fatigue and thermal stress, but also the impact of wear and adhesion arising from the most of the side pressure produced by the pressure action of the gas eruption during combustion. Surface treatment of steel ring has become an indispensable process technology. The common method is ion nitriding treatment for the steel ring to form a wearable nitride layer. Due to the significant improvement of the hardness of the edge area and the attendant special nitrogen precipitates, the wear and tear of cylinder sleeve of the piston ring is greatly reduced and good coordination of the side of piston ring and the side of the ring groove is improved. However, the wearability and relatively low thermal load capacity of the nitriding steel ring are generally insufficient for the first piston ring used in the modern engine.
The most recent generation of piston ring coating is made by the physical vapor deposition method (PVD). Currently, the mainstream surface treatment method is depositing nitride (TiN, CrN, etc.) on the outer round working face of the piston ring. The performance characteristics of the formed coating is high hardness (between 1000 and 2000 HV) and the crystal tissue of ceramic, thereby the coating exhibits low wear and tear rate and high chemical stability. But this coating process derived from the thin film make the application of PVD coating on the piston ring limited. It has been found that when the coating has a thickness of more than 50 μm, adhesion and cracking problems of the coating will occur due to the extremely high inner stress of the coating.
To improve the efficiency and lifetime of piston systems, abrasion-resistant coating with high hardness is gradually widely used. However, although hard coatings can significantly improve the wear life of the friction pair of cylinder sleeve and piston ring, it still does not affect the friction coefficient of their relative movement much. There are effective methods of amorphous carbon coating with the characteristics of both low friction and low wear and tear, such as DLC (diamond like carbon) or GLC (graphite-like carbon). This coating not only has high hardness (−20 GPa), but also has extremely low friction coefficient (<0.1) under oil environment especially the lean oil conditions, which makes it the most promising material of friction reduction and anti-wear of the engine system. However it cannot completely solve the problems of high temperature stability, coating brittleness and long-lasting abrasion resistance.